PSI - Issue 2_B

Matei-Constantin Miron et al. / Procedia Structural Integrity 2 (2016) 3593–3600 Author name / Structural Integrity Procedia 00 (2016) 000–000

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have an inner diameter of 70 mm and an active length of 150 mm. At the ends of the sample, glass fiber tabs were laminated having the thickness of 5 mm and a ply drop-out angle of 6.5°. In terms of wall thickness, two types of specimens were tested: a thin walled specimen having only one braided layer, of thickness 0.35 mm; and a thick walled specimen having three braided layers and the total thickness of 0.95mm. The specimens showed different failure mechanisms depending on the wall thickness. The initial failure of the thin walled specimens is caused by excessive warping leading to local instability of the specimen. Upon reaching critical load the structure buckles and shows no post-critical hardening effects. Secondary to the instability growth the matrix material fails leading to cracks and final failure. The failure of the thick walled specimens (3 layers of braided carbon fiber), is caused by the failure of the matrix material and of the carbon fiber yarns leading to macroscopic cracks going through the thickness of the tested sample. The tested samples show a slight post-critical hardening in the region between the critical torque and the maximum torque.

Fig. 1. (a) Torsion results for 1 Layer braided tubes; (b) Torsion results for 3 Layer braided tubes.

3. Analytical and numerical methods used to predict braided composite behavior 3.1. Analytical prediction of the braided composite’s stiffness

A script written in Mathematica was developed in order to determine the stiffness properties of a braided composite material using the Stiffness Averaging Method (SAM) which considers the behavior of an RUC. This RUC consists of two or three superposed yarn configurations depending on the braiding architecture modeled (biaxial or triaxial). The script takes as parameters the braiding angle, yarn cross-section geometry, yarn-path undulation, fiber volume ratio of the layer and layer topology (biaxial or triaxial braid). The unit cell of a triaxial braided layer is being divided into three different unidirectional yarn configurations, each having its own orientation with respect to the braiding direction and its own undulation path. The longitudinal modulus and the transversal Poisson’s ratio are determined according to the rules of mixture (eqs. 1, 3, 7), Reuss (1929), Voigt (1889); the transversal elasticity modulus, in-plane Poisson’s ratio and shear moduli are expressed using the semi empirical method proposed by Puck (eqs. 2, 4, 5, 6), Puck (1967). (1 ) 1 11      m f E E E (1)

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